The invention relates to an arrangement in a recovery boiler into which spent liquor to be incinerated and combustion air are supplied, the arrangement comprising superheaters in the upper part of the recovery boiler, a boiler bank located in a flue gas duct part known as the second pass immediately after the recovery boiler, the boiler bank being formed of a plural number of parallel elements transverse in relation to the flue gas duct in a size substantially equal to the width of the flue gas duct, and at least one economizer for recovering heat in a duct part after the second pass in the recovery boiler flue gas duct.
Black liquor created in pulp production is incinerated in a recovery boiler at a pulp mill in order to recover from the liquor both heat energy and chemicals that can be recycled in the pulp mill process. Combustion in the furnace generates heat, which is recovered by means of walls and other heat recovery surfaces. These heat recovery surfaces are interconnected in different ways so that different heat recovery surfaces, such as boiler banks and economizers, in the recovery boiler and in the gas duct located after the boiler can be used for heating cold water and converting it into steam, the superheaters then enabling the saturated steam to be heated so that superheated high-pressure steam is produced for the pulp mill and for generating electricity.
Superheaters are usually located in the upper part of the recovery boiler, above what is known as a nose, in a space through which flue gases flow from the recovery boiler to the flue gas duct. The flue gas duct is formed of a plural number of successive at least partly vertical duct parts in which the flue gases flow alternately from top to bottom and from bottom to top. To enhance the efficiency of the boiler, these ducts are usually provided with a boiler bank in the first part, i.e. the second pass, the flowing flue gas heating the boiler bank and thereby bringing water contained therein to boil, the mixture of water and steam then moving into a drum belonging to the water circulation of the recovery boiler. Some of the other flue gas duct parts are typically provided with economizers for using the cooled flue gases to heat cold water supplied to replace the steam that has left the water system of the recovery boiler.
Although the number of economizers may vary according to application, an essential aspect is that a water temperature as high as possible is obtained at the operating temperature of the recovery boiler before the water is conveyed into the drum.